IPCC Fourth Assessment Report

• 1. The Report Structure And Scope

  ›

  In 2007, the Intergovernmental Panel on Climate Change released its Fourth Assessment Report. This document stands as the largest and most detailed summary of climate change ever undertaken. Thousands of authors, editors, and reviewers from dozens of countries contributed to its creation. The process took six years to complete. Over 130 countries provided contributors for this massive undertaking. More than 2,500 scientific expert reviewers examined the drafts. Eight hundred contributing authors wrote sections of the text. Four hundred fifty lead authors coordinated the final work. The report cites over 6,000 peer-reviewed scientific studies. It consists of four principal sections covering physical science, impacts, mitigation, and a synthesis.

  13469

• 2. Physical Science Observations

  ›

  Working Group I published its full report in March 2007. A Summary for Policymakers appeared earlier that February. The section assessed current scientific knowledge of natural and human drivers of climate change. Carbon dioxide levels reached 379 parts per million in 2005. Methane concentrations hit 1,774 parts per billion during the same year. These values far exceed pre-industrial ranges recorded over the last 650,000 years. Eleven of twelve years between 1995 and 2006 ranked among the top twelve warmest years since records began in 1880. Global average temperature increased by about 0.74 degrees Celsius over the past century. Ocean temperatures rose to depths of at least 3,000 meters. Average Arctic temperatures increased at almost twice the global average rate in the past 100 years. Mountain glaciers and snow cover declined on average across both hemispheres. Sea level rose at an average rate of 1.8 millimeters per year from 1961 to 2003. That rate doubled to 3.1 millimeters annually between 1993 and 2003.

  61216

• 3. Impacts On Natural Systems

  ›

  The IPCC convened in Bangkok on the 30th of April 2007 to discuss draft summaries. Agreement emerged among some 2,000 delegates at a meeting held May 4. Stabilization of greenhouse gas concentrations is possible at reasonable cost. Concentrations between 445 parts per million and 535 parts per million would reduce global GDP growth rates by less than 0.12 percent. Renewable energy could provide 30 to 35 percent of electricity by 2030 if carbon prices stay below US$50 per ton. Nuclear power might rise from 16 percent to 18 percent of generation during that period. Carbon pricing schemes ranging from five to 65 dollars per ton are envisioned for 2030. Prices between 15 and 130 dollars per ton appear necessary by 2050 to stabilize around 550 parts per million. New buildings could reduce emissions considerably despite barriers in developing countries. Energy efficiency improvements often prove more cost effective than increasing supply. Net additional investment required ranges from negligible to five to ten percent of current spending. Government funding for

  5721

• 4. Mitigation Strategies And Costs

  ›

  most energy research programs has declined to about half the 1980 level over nearly two decades.

  Model-based scenarios predict temperature rises between 1.8 degrees Celsius and 4.0 degrees Celsius by 2100. These projections depend on different emission pathways chosen by human societies. A low scenario estimates a best estimate rise of 1.8 degrees with a likely range of 1.1 to 2.9 degrees. High scenarios project a best estimate increase of 4.0 degrees with a possible range up to 6.4 degrees. Sea level rise in a low scenario spans 18 to 38 centimeters. The high scenario projects increases between 26 and 59 centimeters. Arctic late-summer sea ice disappears almost entirely in some projections by the latter part of the 21st century. Approximately 20 to 30 percent of species assessed face increased extinction risk if warming exceeds 1.5 to 2.5 degrees Celsius. Model projections suggest significant extinctions of 40 to 70 percent when global average temperature increases exceed 3.5 degrees Celsius. Partial loss of polar ice sheets could imply meters of sea level rise over millennial time scales. Rapid sea level rise on century time scales

  82325

• 5. Future Climate Projections

  ›

  cannot be excluded from current models.

  A paragraph in the Working Group II report projected Himalayan glaciers could disappear by 2035. This statement appeared in chapter 10 of the original draft but was removed from the final Summary for Policymakers. The IPCC acknowledged that date as incorrect while reaffirming the overall conclusion remained robust. Rajendra K. Pachauri responded to criticism during an interview with Science magazine. Former chairman Robert Watson stated mistakes seemed to make climate change appear more serious than reality. Martin Parry called the situation a clamor without substance after investigating other alleged errors. The error originated from a misquote within a World Wildlife Fund report. That source had itself misquoted an ICSI report about snow and ice variations. The IPCC expressed regret for poor application of established procedures in this instance. Two errors were found regarding melting glaciers and Dutch land area below sea level. Skeptics used these findings to question the entire assessment process despite scientific consensus remaining strong.

  The New York Times reported that global warming is unequivocal according

  69131514

• 6. Controversies And Corrections

  ›

  to leading international scientists. Human activity causes very likely most temperature rises since 1950. The Associated Press summarized positions on sea level rise and extreme weather events. Several science academies reiterated conclusions from the Fourth Assessment Report. Joint statements emerged in 2007, 2008, and 2009 by Brazil, China, India, Mexico, South Africa, and G8 nations. The Australian Academy of Science published supporting documents. A joint statement came from the Network of African Science Academies in 2007. The Inter Academy Medical Panel issued a statement signed by 43 scientific academies in 2010. The United States National Research Council concluded climate change poses significant risks to human and natural systems. The Netherlands Environmental Assessment Agency carried out two reviews generally supportive of AR4 conclusions. Forty-six countries called for creation of a United Nations Environment Organization during a Paris Call for Action read by French President Chirac. That group included European Union nations but excluded top emitters like the United

  2235365152

• 7. Global Reception And Legacy

  ›

  States, China, Russia, and India.

  3536394041